A power converter is a power processing circuit that converts an input voltage waveform into: a specified output voltage waveform. In many applications requiring a DC output, switched-mode DC/DC power converters are frequently employed to advantage. The switched-mode DC/DC power converters generally include an inverter, an input/output isolation transformer and a rectifier on a secondary side of the isolation transformer. The inverter generally includes a main power switch, such as a field effect transistor (“FET”), that converts the DC input voltage to an AC voltage. The input/output isolation transformer, then, transforms the AC voltage to another value and the rectifier generates the desired DC voltage at the output of the power converter. Conventionally, the rectifier includes a plurality of rectifier switches (e.g., diodes, or FETs acting as synchronous rectifier switches) that conduct the load current in response to the input waveform thereto.
The main power switch and rectifier switches are usually operated at relatively high switching frequencies such as 200-300 kHz to allow the use of smaller components such as inductors and capacitors within the power converter. As a result, parasitic or stray inductance or capacitance associated with the components of the power converter can be reduced.
The residual parasitic elements mentioned above, however, may generate high frequency oscillations that appear as undesired “ringing” waveforms in the power converter associated with the switching transitions, particularly, those associated with the transformer and switches. The ringing waveforms, which are superimposed on the waveforms associated with the normal operation of the power converter, prompt the use of higher rated and higher cost circuit components to operate in such an environment. Additionally, the deleterious ringing waveforms cause the power converter to be more lossy and less efficient. Some of the loss manifests itself as undesirable electromagnetic interference (EMI) causing regulatory problems which must be addressed. Due to the relatively small resistance values inherent in the transformer and inductor elements, the ringing energy may only be lightly damped in the power converter.
The spurious ringing necessitates that a rectifier switch with a higher peak inverse voltage rating be employed in the power converter. For example, it the rectifier switch is employed in a power converter with a steady state output voltage of about two to three volts and, during a transition, the rectifier switch endures a reverse voltage spike causing perhaps a 80-90% rise in the off-state voltage of the rectifier switch, then the rectifier switch must be rated for roughly twice the peak inverse voltage rating (e.g., 30 volts) that it would otherwise require to avoid being damaged.
This problem is particularly severe for rectifier switches that conduct during an auxiliary conduction period (i.e., when the main power switch is not conducting; also referred to a “reset” portion of the switching cycle) of the power converter. During this period, the reverse voltage sustained by the rectifying switch is the input voltage of the power converter multiplied by the turns ratio of the transformer, which can be unavoidably high for higher input voltages. In addition, when the higher reverse voltage is superimposed on the non-zero voltage turn-on of the main power switch, the power converter is subject to voltage stresses that may further compromise the design thereof.
While employing a rectifier switch with a 30 volt peak inverse voltage rating in a two to three volt power converter may be acceptable because of the scarcity of commercially available and economical rectifier switches with lower voltage ratings, the problem is exacerbated in power converters with a higher steady state output voltage (e.g., 24 or 48 volts). In such circumstances, the rectifier switch may be subject to a reverse voltage spike of 200 volts or more. While it is possible to employ rectifier switches with higher peak inverse voltage ratings (even with the higher cost and poorer performance characteristics), the benefits of exploring other circuit alternatives outweighs accepting such losses and poor efficiencies associated with using such devices.
Conventional ways of reducing the spurious ringing in the power converter include a snubber circuit placed across each rectifier switch which consists of, in one example, a resistor connected in series with a capacitor. The snubber acts as a damping device to reduce the ringing amplitude by dissipating a portion of the ringing energy. While the snubber circuit reduces the effects of the spurious ringing associated with the rectifier switch allowing lower rated devices to be used, it also reduces the overall efficiency of the power converter. More specifically, the snubber capacitor causes more current to flow through the switches of the power converter when it conducts providing additional energy losses therein. Analogous drawbacks are provided by other passive snubber approaches such as circuits employing a diode in series with a capacitor to absorb the reverse voltage spike, and a resistor to dissipate the energy accumulated in the capacitor.
A further technique for reducing the ringing waveforms in the power converter is to place a saturable reactor in series with the rectifier switch. The saturable reactor is a nonlinear inductor that adopts a lossy characteristic change as the current therethrough increases to a point where the magnetic core material saturates. The saturation characteristic can damp the ringing waveforms by dissipating some of the ringing energy (and reducing the EMI), but it tends to become physically hot and, as a result, is often impractical to use in the power converter.
Other damping circuits such as active snubber circuits may also be used in a variety of schemes to reduce the ringing waveforms. Examples of active snubber circuits are illustrated and described in L. H. Mweene, et al., A 1 kW, 500 kHz, front-end converter for a distributed power supply system, Proc. IEEE Applied Power Electronics Conf., March 1989, pp. 423-432; R. Redl, et al., A novel soft-switching full-bridge dc/dc converter: analysis, design considerations and experimental results at 1.5 kW, 100 kHz, IEEE Power Electronics Specialists Conf. Rec., 1990, pp. 162-172; G. Hua, et al., An improved zero-voltage-switched PWM converter using a saturable inductor, IEEE Power Electronics Specialists Conf. Rec., 1991, pp. 189-194; K. Harada, et al., Switched snubber for high frequency switching, IEEE Power Electronics Specialists Conf., 1990, pp., 181-188; V. Vlatkovic, et al., High-voltage, high-power, ZVS, full-bridge PWM converter employing an active snubber, Proc. IEEE Applied Power Electronics Conf., March, 1991, pp. 158-163. The aforementioned references are incorporated herein by reference.
Still further examples employing active snubber circuits including switches coupled to the windings of the transformer to reduce the ringing waveforms are described in U.S. Pat. No. 5,636,107 entitled DC—DC Converters, by Lu, et al., U.S. Pat. No. 5,781,420 entitled Single ended forward DC-to-DC converter providing enhanced resetting for synchronous rectification, by Xia, et al., U.S. Pat. No. 5,986,899 entitled Single ended forward DC-to-DC converter providing enhanced resetting for synchronous rectification, by Xia, et al., U.S. Pat. No. 6,141,224 entitled Single ended forward DC-to-DC converter providing enhanced resetting for synchronous rectification, by Xia, et al., U.S. Pat. No. 6,278,621 entitled Single ended forward DC-to-DC converter providing enhanced resetting for synchronous rectification, by Xia, et al., which are incorporated herein by reference. Lu, et al. disclose a series-coupled switch and capacitor that clamps a voltage across the secondary winding of the transformer. Unfortunately, the effectiveness of Lu, et al. is limited because portions of the active snubber circuit are in series with ones of components of the power converter to be protected. This limitation is especially pronounced for reverse voltages exhibiting wider pulses. While the Xia, et al. references disclose active snubber circuits located across the rectifier switches, these circuits are typically enabled during the auxiliary conduction period and principally act to reset the flux in the core of the transformer.
While presently available active circuits analogous to the active snubber circuits have been employed with the main power switch of the inverter of the power converter (see, for instance, U.S. Pat. No. Re 36,098, entitled Optimal Resetting of the Transformer's Core in Single-ended Forward Converters, by Vinciarelli, which is incorporated herein by reference), such techniques have not been applied directly to a rectifier switch that substantially conducts during the auxiliary conduction period employing a control voltage from the transformer, thereby allowing the efficient use of a rectifier switch with a lower voltage rating and with fewer circuit components.
Accordingly, what is needed in the art is a robust solution and circuit to reduce the undesirable ringing waveforms associated with the rectifier switch in the power converter without significantly affecting the efficiency thereof.